To cope with the regulation for emission of carbonic acid gas and energy conservation, thermoelectric power generation utilizing thermoelectric conversion of converting waste heat directly into electricity is attracting attention. This thermoelectric conversion is effected by such a mechanism that when temperature difference is generated by assigning one end of n-type and p-type semiconductors to a high temperature and another end to a low temperature, potential difference is caused and electricity is obtained. The principle thereof has long been known.
For example, JP-A-2001-189495 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), WO03/019681 A1, JP-A-2004-253618, JP-A-2004-119647, Yamanaka et al., Kinzoku (Metals), Vol. 74 (8), page 54 (2004), Matsuura et al., Journal of Japan Institute of Metals, Vol. 66 (7), page 767 (2002), and S. Shuto et al., Proc. 22nd International Conference on Thermoelectrics, 312 (2003) disclose various types of thermoelectric conversion material.
Conventional thermoelectric power generation had been used only for limited uses such as application to space probe, because the semiconductor used for the power generation is very expensive, but in the late 1990s, a high-performance semiconductor was newly discovered and since then, aggressive development such as study on materials for practical use, production of modules and packaging test has been made.
Examples of the substance taken notice of as a next-generation thermoelectric semiconductor include filled skutterudite type, cobalt oxide, silicide and Heusler type. It is recognized that with these materials, high electric conductivity, high Seebeck coefficient and low thermal conductivity can be achieved at the same time. For enhancing the performance of each material, a great deal of effort is being made.
Along with elucidation of properties as a semiconductor, alloys having a Heusler or half Heusler structure have come to draw attention as an excellent thermoelectric semiconductor.
The Heusler alloy is represented by the formula: A3-XBXC, wherein A and B are each a transition metal, C is a Group III or IV metal, and a space group is Fm3m. The half Heusler alloy is represented by the formula: ABC, wherein A and B are each a transition metal, C is a Group III or IV metal, and the space group is F43m.
In the case of using a Heusler alloy as a thermoelectric semiconductor, the design is difficult due to numerous combinations of elements, and one proposed guideline therefor is a method of using an electronic number as a rough standard.
With respect to the thermoelectric semiconductor having a Heusler structure, for example, Nishino et al. have reported an Fe2VAl system of giving an output factor comparable to that of Bi—Te system in the vicinity of room temperature. The Fe2VAl system is expected in view of the theoretical value to exhibit thermoelectric performance higher than the Bi—Te system and also from the aspect of expensiveness or toxicity, is noteworthy as a practical material.
At present, for the production of a half Heusler alloy as a thermoelectric conductor for use in middle and high temperature regions, a heat treatment for a long time of about 10 days is performed. Considering mass production, the heat treatment for such a long time is not preferred because this causes increase in the cost.
Also, it is known that in the TiNiSn system having a half Heusler structure, when Ti is replaced by Zr or Hr, both high output factor and low thermal conductivity can be achieved in a high temperature region of 300° C. or more and the dimensionless figure of merit ZT as a rough standard for the performance of a practical material exceeds 1.0 at 693K. The performance of this system is expected to be enhanced in future.
When a Heusler alloy is produced by casting, high melting point metals such as Ti, V and Zr are contained therein in many cases and a skilled technique is required for the quench-solidification of high-temperature molten metal. Also, a casting technique in vacuum induction furnace and inert atmosphere is necessary because of handling of a readily oxidizable metal as represented by Ti.